Polybenzimidazoles are a class of polymers characterized by a high degree of thermal stability.
The preparation of poly[2,5(6)-benzimidazole], ##STR1## by heating phenyl 3,4-diaminobenzoate has been described by Marvel et al. in Example 1 of U.S. Pat. No. 3,174,947 (Reissue 26,065). This reference also describes the preparation of other polybenzimidazoles from various o-diaminocarboxylic acids in the form of their phenyl esters, as well as from mixtures of monomers in which one compound contains a pair of o-diaminoaryl substituents and the other compound is a diphenyl ester of an aromatic dicarboxylic acid.
Japanese Patent Application No. 18,352/67, describes a process for preparing poly[2,5(6)-benzimidazole] by heating 3,4-diaminobenzoic acid in the presence of polyphosphoric acid. Although it is desirable from a commercial viewpoint to prepare the polymer directly from 3,4-diaminobenzoic acid, rather than from its phenyl ester, it has been found difficult to reduce the bulk polymer so prepared to the small particles required for dissolution. It would be desirable for the polymeric product to be prepared in the form of small particles so that polyphosphoric acid and other impurities could be easily removed and the particles easily dissolved to low solutions suitable for extrusion into shaped articles. A further problem is that the reaction mass employed in this process is quite corrosive towards conventional steel reaction vessels, and a less corrosive process would be desirable.
Polybenzimidazoles may be prepared in the form of fibers and other shaped articles which are flame resistant and which can be used for many purposes in high-temperature environments. For instance, polybenzimidazole fibers are suitable for use in making filter bags for removal of particles from hot stack gases, owing to the stability of the fibers in hot acidic environments.